It is desirable to prevent unauthorized or counterfeit reproduction or forgery of many types of original documents. Such documents may include paper currency, negotiable instruments, event tickets, official records, medical prescriptions, diplomas, and many others.
As copier technology has improved, it has become easier to make realistic-looking copies of many of these original documents. In many cases, a copy can be difficult to distinguish from the original. In response, producers of these documents have added features to documents that make them harder to copy. These features often take the form of a security background printed on the original document by a high resolution printing process such as offset printing. Offset printing is typically an analog printing operation performed at a resolution that is equivalent to between 2,400 and 10,000 dots per inch (dpi). This security background on the original document is substantially indistinguishable to the naked eye at a normal viewing distance. However, a typical copier has scanning and printing capabilities that are of lower resolution than that of offset printing, often in the range of 300 to 1200 dpi. In addition, the optical scanner of a typical copier perceives and captures the security background differently from the human eye. As a result, the security background is readily detectable by the human eye on a reproduced document.
Nonetheless, in many applications it is desirable to print original documents on digital printing systems that are of lower resolution than offset printing, and of comparable resolution to copiers. Offset printing typically has high setup costs, and thus is cost-effective for printing large quantities of the identical document. However, many original documents are not printed in large quantities, and often original documents are printed in single quantity. For example, even if a set of diplomas for a particular university is printed in a single print run, the name of the graduate on each diploma will be different. Thus digital printing systems would be better suited to such applications than offset printing systems.